1. Field of the Invention
The invention relates to a flow meter for pulmonary function tests, in particular to a Pitot tube flow metering device for pulmonary function tests.
2. Background Art
In everyday life, a great number of apparatuses for pulmonary function tests based on various operation principles facilitate the work of physicians in recognizing organic changes and/or defects of respiratory tracts, and/or when testing and monitoring respiratory functions. A class of apparatuses for pulmonary function tests is constituted by devices of direct construction (e.g. spirometers, rhinomanometers, whole body plethysmographs, diffusing capacity meters, ergospirometers) that involve a flow meter as the primary measuring transducer. Various means/techniques, such as e.g. means and processes based on the detection of change in ultrasonic waves or equipments and techniques based on the pressure difference measurements in various geometrical arrangements (i.e. employing different flow resistances) are made use of as the flow meter and/or to meter the air stream (induced, in general, by the respiration of the individual examined) flowing through the apparatus.
The Pitot or Prandtl tube equipments used to determine the difference between the static and stagnation pressures induced by an air stream are, in particular, based on a pressure difference measurement. The essence of such an equipment is that a double-wall probe is arranged, generally, within the measuring tube, by means of which the static and stagnation pressures created by the flow stream can be simultaneously measured. Then, based on Bernoulli's law, the magnitude of the unknown flow rate can be unambiguously determined. As the general advantage of the Pitot or Prandtl tube equipments, their relatively simple construction, small dead space and low flow resistance can be mentioned. Their great disadvantages are, however, that the useful signal generated by them is relatively noisy (i.e. in this case the signal-to-noise ratio influencing the precision and reliability of the measurement is not satisfactory) and/or their characteristics is nonlinear (quadratic).
U.S. Pat. No. 5,088,322 discloses an averaging Pitot tube flow measuring device for the measurement of a gas flow, in particular, a gas flow emerging due to exhalation/inhalation. Pressure lead-out elements of the Pitot tube are made in the form of radial pressure offtake vanes at the angle of 120° to one another. The surface of said vanes which is in contact with the gas flowing through the measuring tube is provided with channels suitable for leading away stagnation pressure. The channels extend along the full length of each vane, specifically from the inner surface of the wall of the measuring tube to an aperture located on the longitudinal axis of said measuring tube. Accordingly, for a certain vane, the averaging of pressure values takes place radially from the center of the measuring tube to the wall thereof. The arrangement of said vanes in which they are at the angle of 120° to one another enables the averaging of pressure values along the directions X and Y as well (here and from now on, direction Z corresponds to the direction parallel with the longitudinal axis of the measuring tube). Transmission of the pressure signal to a transducer arranged outside of the measuring tube takes place through said central aperture. In one of the embodiments, the vane surfaces provided with channels are located in a plane perpendicular to the longitudinal axis of the measuring tube. In a possible further embodiment, the vane surfaces extending from the inner surface of the measuring tube to the axis of symmetry thereof incline in the radial direction. A flow meter according to U.S. Pat. No. 5,088,332 is apt for measuring flow rates of either direction (i.e. in case of either exhalation or inhalation; symmetric construction). Said flow meter can also be used to analyze the gas composition flowing through the measuring tube. An outlet for the analysis of the gas composition is located symmetrically with respect to the central pressure offtake aperture, that is the measurement of both the pressure and the gas composition can be performed under the same conditions even if the flowing is of opposite direction.
The major disadvantage of said flow meter is that the pressure offtake aperture is located just at the geometrical center of the measuring tube, wherein the flow rate is the highest. Consequently, the risk of being contaminated by the minute moisture and/or the drops of sputum exhaled with air through getting into the offtake aperture is also the highest just at this point. As the flow meter at issue was primarily developed to monitor air breathing apparatuses, an appropriate filtering member is always incorporated into the total respiratory circuit when it is actually used for this purpose. However, in such an arrangement, the flow meter according to U.S. Pat. No. 5,088,332 is suitable for pulmonary function tests only in a limited manner. To avoid contamination through moisture and sputum, a surface coating capable of adsorbing water is optionally applied onto the vanes. When the flow meter is manufactured, this always means at least one further production step to be performed. This, as well as the coating applied itself increase the production costs of said flow meter. Moreover, the coating used encumbers significantly the recycling of the contaminated measuring tube.
International Publication Pamphlet No. WO 03/047429 teaches a symmetric and averaging Pitot tube respiratory flow metering device, as well as various pressure averaging techniques. Here, the Pitot tube is realized with a sensing tube arranged within a measuring tube, wherein said sensing tube is perpendicular to the direction of flow and crosses the longitudinal axis of said flow metering device. Sampling of pressure takes place through sensing bores formed on the sensing tube at its given points along the direction of the radius of the measuring tube. A disadvantage of said flow metering device is that it is apt for averaging only along one of the directions X and Y at a time. Another disadvantage of the device at issue is that the sensing tube cannot be removed together with the measuring tube after every single measurement and, hence, there is a possibility of cross infection between individuals examined one after the other if disinfection of said device is inappropriate. A further disadvantage of the flow metering device concerned is that the sensing bores used for the sampling get easily clogged (because of their sizes) due to the moisture and/or sputum content of the exhaled air. This holds especially for diagnostic inspections of the lung.
The flow meter head according to Polish Patent No. 173,767 also represents essentially a symmetric and averaging Pitot tube flow metering device. A disadvantage of said device is that the aerodynamical resistance forming a part thereof only measures a vertical segment of the flow traveling through the measuring tube, that is, it averages only along a single direction. Another disadvantage of the device concerned is that the channels within said resistance directly guide the moisture/sputum condensing within the measuring tube into the pressure gauge sensing means.
U.S. Pat. No. 5,038,773 discloses a further symmetric and averaging Pitot tube flow metering device that is basically made use of as a disposable flow meter. In this device, bores for pressure sampling are formed in the ends of two ribs located in the vicinity of the inner surface of the measuring tube wherein said ribs are crossing each other and arranged in the path of flow within said measuring tube. As a consequence—and accordant to the above—the risk of being contaminated decreases, since at the positions of the sampling bores the velocity of an air stream flowing through the measuring tube becomes smaller. In turn, when said flow meter is to be used for pulmonary function tests, said bores can easily be clogged by the moisture and/or sputum content of the exhaled air due to their small cross sections. A further disadvantage of said flow metering device is that the pressure sensing bores formed in the vicinity of the ends of the ribs do not enable radial averaging.
As it has been mentioned earlier, a common disadvantage of the Pitot tube flow metering devices is that their signal-to-noise ratios are dissatisfactory and should be enhanced in most cases. In Pitot tube measuring techniques and flow meters the enhancement of the signal-to-noise ratio is performed on electric signals obtained by transducers and corresponding to the pressure signals by either electronic circuits representing a low-pass filter or by means of appropriate softwares. Increase in the signal-to-noise ratio takes place, in general, relatively far away from the place where the pressure signals were actually generated, and thus said pressure signals can be further distorted during their propagation. As a consequence of this, the precision and dynamics of the measurement degrades.
U.S. Pat. No. 5,048,327 teaches a Venturi tube gas flow metering device to be used for governing the fuel injection system of internal combustion engines, wherein the signal-to-noise ratio of the difference signal of a high pressure and a low pressure is increased by means of a low-pass pneumatic filtering arrangement. To this end, before the operation of comparison, said high pressure is guided through a conduit representing a flow resistance R and a ring-shaped reservoir representing a capacitance C, filtering out the pressure fluctuations present in the high pressure thereby. Said U.S. patent is expressedly limited to Venturi tube flow meters, the applicability of a low-pass pneumatic filtering arrangement in other type of flow meters is not mentioned (and is not even hinted at). Furthermore, said document contains no teaching as to the adaptation of said filtering arrangement for flow meters based on other principles.